The exhaust gas emitted from an internal combustion engine is a heterogeneous mixture that may contain gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
In the realm of exhaust treatment technologies, there are several known filter structures that have displayed effectiveness in removing particulate matter from the exhaust gas such as ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications.
Typically, a particulate filter is disposed within an exhaust treatment system to filter particulates from the exhaust gas. Over time, the particulate filter may become full and cleaning or “regeneration” is required to remove the trapped particulates. Regeneration of a particulate filter in vehicle applications is typically automatic and is controlled by an engine or other controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the particulate filter to levels that are often in the range of 600° C. in order to burn the accumulated particulates and to thereby enable the continuation of the filtering process.
There are certain control issues associated with exhaust gas filter regeneration. A drop-to-idle condition, which occurs when an internal combustion engine operating at moderate to high speed suddenly slows to a low or idle speed, may occur during regeneration. This condition may result is a sudden reduction of the exhaust gas flow rate through the regenerating filter. Upon a sudden reduction in exhaust flow through the regenerating filter, the heat generated by the oxidation of the accumulated particulates accumulates in the particulate filter (i.e. the exhaust gas flow rate is insufficient to remove the heat from the oxidizing particulates) and may lead to a temperature excursion that can be damaging to the filter substrate.
Accordingly, it is desirable to provide an apparatus and method for controlling temperature excursions that may occur during drop-to-idle engine operation during a particulate filter regeneration event.